In recent years, from the aspect of protecting the earth environment, the automobile and motorbike industries have been making material plates thinner year by year for weight reduction to improve fuel efficiency. Accordingly, increasing productivity and welding quality is requested in arc welding for thin plates by robots. Then, demands for increasing welding speed, decreasing sputtering, and preventing defects (e.g. burn-through and undercut) need to be satisfied. Among them, increasing welding speed increases the number of products manufactured per unit time. Meanwhile, preventing burn-through for workpieces having a gap therebetween raises yields of object to be welded to reduce reworking worker-hours. Under the circumstances, the market has been requesting solution to the problems more strongly year by year.
To satisfy these requests, various methods for increasing welding speed and for gap welding have been devised conventionally. For example, the following method is available. That is, a welding output current immediately after arc regeneration is made higher than that immediately before short circuit opening to cause a droplet to be formed soon after the arc regeneration, which shortens the cycle of short circuit generation, lowers welding voltage, and shortens the arc length (refer to patent literature 1 for example). This method suppresses burn-through even in gap welding by increasing welding speed to lower heat input.
FIG. 4 shows waveforms of welding output voltage and a welding output current in a case where welded by the above conventional output control method. In FIG. 4, the horizontal axis indicates elapsed time, and the vertical axis indicates welding output voltage and a welding output current. In FIG. 4, short circuit period 101 is in a state where a wire and base material short-circuit. Arc period 102 is in a state where an arc is generated between the wire and base material. At arc regeneration time point 103, the short circuit opens to regenerate an arc. Immediately before the short circuit opening, immediately-before-short-circuit-opening current 104a is flowing. In arc period 102, arc initial current 105a is flowing during initial control period 106.
Next, the current waveform shown in FIG. 4 is described in relation to a control way by elapsed time. After an arc is regenerated, a low welding output current is steeply increased to be arc initial current 105a. Control is exercised so that a constant current is output during initial control period 106, and then the welding current value gradually decreases by arc control (voltage control). As shown in the drawing, arc initial current 105a is controlled for a value of immediately-before- short-circuit-opening current 104a with superimposed current value 107 added thereto, which enables a droplet to be formed early at the tip of the wire. In other words, arc initial current 105a is controlled so as to be always higher than immediately-before-short-circuit-opening current 104a. 
This enables a droplet to be formed soon after the arc regeneration, which shortens the cycle of short circuit generation, lowers welding voltage, and shortens the arc length. Accordingly, the welding speed is increased to lower heat input, thereby suppressing burn-through even in gap welding.
In the above-described conventional output control method, however, longer arc length than appropriate one due to such as a disturbance causes arc period 102 to be extended as shown in FIG. 5. Consequently, the cycle of short circuit generation becomes longer than that in the welding state shown in FIG. 4, thereby raising the average value of welding output voltage. In this case, the meltage of the wire increases to extend the next short circuit time, which causes current value 104b immediately before the short circuit opening to be longer than current value 104a immediately before the previous short circuit opening. Further, arc initial current 105b becomes longer than previous arc initial current 105a. 
The higher average value of welding output voltage requires lower output voltage. However, arc initial current 105b is controlled so as to be higher than previous arc initial current 105a, and thus control is exercised so as to increase welding output voltage. That is to say, the short circuit cycle cannot be shortened by suppressing the meltage to shorten the arc length.
Accordingly, when a disturbance (e.g. extended projection length and arc length) occurs, the meltage of the wire increases and the arc period is extended, which causes the bead width to be uneven. Further, heat input cannot be reduced, which causes such as burn-through in gap welding.